With present concerns over global warming, carbon emissions and limited access to or availability of traditional fossil fuels, sources of alternative, renewable, energy are becoming more and more important.
Solar, wind and wave power systems for generating electrical energy are well known. Solar thermal energy systems work well when there is sufficient sunlight, otherwise energy needs to be stored during sunlight hours and released overnight if continuous energy output is required. This is often achieved through phase change salts which liquefy when storing thermal energy and re-solidify when that thermal energy is released. However, if insufficient thermal energy cannot be stored during daylight hours, there is a risk of energy output being exhausted.
Wind energy obviously requires sufficient movement of air to generate power form wind turbines. Whilst wind turbine technology has progressed and wind energy has become more cost effective, there remains the challenge of intermittency and of siting multiple wind turbines sufficient to generate the required amount of renewable energy. Wind turbines create a visual and noise impact on land that is not always acceptable to locals. Wind farms have more recently been sited out at sea in order to benefit from more reliable prevailing winds and to reduce the environmental impact from the presence of the tall wind turbines.
The extraction of energy from the seas and oceans is an accepted concept in the field of renewable energy. One major benefit of wave energy is that there is almost always at least some energy to be extracted from waves, thereby making wave energy potentially more reliable than solar thermal or wind energy and more environmentally acceptable in the sense of having lower visual, and potentially lower noise, impact on the local environment.
Whilst wave energy is relatively straightforward in concept, wave energy is, however, a complex energy medium.
The energy effectively ‘rolls’ through the seas and oceans, moving water particles in a circular or elliptical (depending on the depth of the water), oscillating motion (up and down “heave”, and back and forth “surge”), relying on the fluidity and viscosity of the water to transmit the energy from one place to another. The water is constantly changing direction both vertically and horizontally and constantly changing in nature from potential (height) to kinetic (motion) energy and combinations in between.
Waves from seas and oceans are also not regular. While there may be an average period and average wave height at a given location and given time, there will still be variations from wave to wave, and within each wave.
This all poses a significant challenge to any wave energy converter (WEC) (sometimes called a wave energy conversion device or WECD) trying to extract the energy from the wave. To be efficient, the converter must ‘couple’ well with the wave (i.e. respond to or ‘track’ the wave motion) while at the same time resist the wave sufficiently to extract energy from the wave. The converter must also be robust to withstand wave forces, particularly adverse forces during storms, and be efficient with low energy losses, and be cost effective.
Many different WECs have previously been conceived. However, development of a cost effective converter that is also sufficiently efficient at extracting energy from the wave and also sufficiently reliable has, as yet, proved elusive.
Many WECs are very large and ‘mechanical’ in their construction and operation, resulting in high manufacturing, maintenance and/or operating costs, as well as difficulties in maintaining coupling with the wave due to the high mass and inertia of the converter. This results in low coupling (bandwidth) and poor real world performance.
Some WECs use relatively complex methods of wave energy capture, transmission and conversion to electrical energy, often progressing through consecutive energy conversions, from wave (fluid) energy to mechanical to hydraulic to rotational mechanical (turbine) to electrical energy. Each stage of conversion has an efficiency and system complexity cost, and leads to energy losses. Each stage of transmission has frictional losses, particularly when working with dense fluids (liquids) under high pressures and velocities such as hydraulics.
Some converters only focus on one directional component of the energy in the water (i.e. the vertical or horizontal component) and do not try to or effectively capture both vertical and horizontal components or accommodate the fluid nature of the energy flow in waves.
Other WECs use the oscillating water column (OWC) principal which relies on moving large quantities of water in and out of an open chamber or chambers, that movement pumping air back and forth over a turbine to turn a generator. These OWC converters often require the water to change direction and flow around non-streamlined edges. This increases friction and energy losses in the system and can introduce undue lag which can prevent good coupling with the wave. These converters also require a considerable amount of material in their construction, installation or anchoring, relative to the power output of the converter. The turbine is also exposed to salt laden air which can increase the cost to resist corrosion and the maintenance cost associated with keeping the blades clean to maintain performance
Many WECs are also located offshore and on the surface of the ocean where extremely high forces are evident during adverse weather conditions. This increases the cost of the converter relative to the effective power output.
One field of WECs that has shown great potential can be characterised as ‘membrane’ power conversion converters, and more particularly membrane—pneumatic power conversion converters. These converters use a series of low cost and low inertia membranes or diaphragms to interact with the wave and efficiently transfer energy to a second fluid, usually a low inertia, low friction fluid such as air, which transmits the energy onto a turbine and electrical generator. These WECs can, in general, exhibit better coupling with the wave than other WECs because of low system inertia (fast response) and due to their reduced complexity, and have the potential to produce power more cost effectively over a broader range of wave conditions than other types of WEC discussed above.
Various wave energy converters are discussed in prior patent documents. For example, U.S. Pat. No. 3,353,787 to Semo in the 1960s proposed using water or oil as the second (transmission) fluid. The objective was to have a sturdy, sub sea converter better able withstand storms and harness a greater proportion of the available wave energy than other more complex converters at the time. Semo proposed a series of elongated chambers each with a flexible upper surface to pump an incompressible fluid (liquid) through check valves to a shore based fluid motor for energy extraction. Flow returned from the shore in the same circuit as the outflow but entered the chambers through small orifices.
U.S. Pat. No. 3,989,951 to Lesster in the mid 1970's discusses a submerged converter using a compressible fluid (such as air) as the transmission fluid to improve the responsiveness of the converter by reducing the mass and inertia of the transmission fluid. Lesster also provided for shorter flexible walled cells to improve the flexibility of operation and had the waves run along the length of the converter over each of the cells in turn. Separate in and outflow circuits, took air from each of the cells via check values, and led to one or more turbo generators in a closed loop circuit providing a ‘push pull’ action on the air flow.
U.S. Pat. No. 4,164,383 to French in the late 1970's maintained the longitudinal (spine) design and orientation of the converter facing perpendicular to the wave front. This converter used a closed loop circuit with check valves and air as the transmission fluid but moved the converter to being a floating converter at or just below the surface of the waves like a long ‘spine’ and used a single flexible enclosure like a bag divided into compartments.
U.S. Pat. No. 4,375,151 to French in the early 1980's later disclosed control systems using the wave height and multiple closed loop circuits and turbo generators to improve the efficiency of energy extraction by reducing airflow pulsations and improve the converter's sea keeping, particularly pitch control.
U.S. Pat. No. 4,441,030 to Bellamy in the early 1980s discloses a similar floating ‘spine’ design but in a ‘termination’ mode i.e. parallel to the wave front, with flexible ‘pillow shaped bags’ mounted off the side of the spine to capture wave energy and reduce bag wear. The primary focus was on the bag design but this patent document also discloses the use of a single self rectifying turbine per cell rather than the previous closed loop circuits.
A later patent document U.S. Pat. No. 4,675,536 to Bellamy then progressed in the mid 1980s to a circular or ring design to reduce the size and cost of the converter, improve its sea keeping, and reverting to the option of using a series of membranes but now vertically positioned, rather than bags to capture the wave energy.
Development of membrane converters then appeared to be stagnant until developments revealed in US 2011-0185721 to Turner and US 2011-0162357 to Bellamy et al addressed the principle further in 2008. The Turner document focuses largely on a circular converter with an “S” shaped mounting edge for their membrane, and on other design features of the membrane (size, thickness, stiffness, reinforcement etc). Bellamy et al also stayed with a circular converter (or ‘endless spine’) but introduced a combination of membranes and oscillating water columns to the one converter. The objective being to increase the effectiveness and ‘bandwidth’ of the converter (i.e. coupling with the wave) by engaging with the both the heave (vertical) and surge (horizontal) components of the wave energy. Bellamy et al also reverted back to the non-return valve, single directional airflow (closed loop circuit) feature of Lesster, French and to some extent Semo.
U.S. Pat. No. 7,554,216 to Winsloe and WO 2007/057013 to Rasmussen both disclose oscillating water column (OWC) converters with multiple cells and a closed circuit airflow system using check valves feeding into a high pressure manifold, onto a turbo generator, and returning via a low pressure manifold. Both converters are floating OWC converters and are fully exposed to adverse wave conditions.
Alternatively, it is desirable of the present invention to provide a wave energy converter (WEC) better able to harness available wave energy than the aforementioned known converters.
With the aforementioned in mind, it is desirable of the present invention to overcome the difficulties of such converters by preferably providing a sub sea wave energy converter better able withstand storms.
Alternatively, it is desirable of the present invention to provide a wave energy converter with improved operational efficiency compared with known wave energy converters.